The present invention relates to an optical disc on which information is recorded, and more particularly to an optical disc provided with annular stack rings on both surfaces thereof for preventing the disc from adhering to another disc when put on the other disc during the production thereof.
A CD which is one of these optical discs, has an information recording surface formed on one side of a substrate made of a transparent resin. Digital signals are recorded in the form of fine depressions called pits on the recording surface. The recording surface is covered with a reflecting film by such method as the aluminum evaporation, and further with a protecting layer made of hard resin formed on the reflecting layer. A laser beam is radiated from the other side (reading surface) of the substrate so that the signals are optically reproduced. In addition, a label regarding the recorded information is printed on the protection layer.
During the manufacturing the CD by the procedure described above, after such steps as evaporation and printing, it is necessary to dry the formed or the printed surface of each disc before proceeding to the next step. In order that the space for manufacturing the CD is efficiently utilized particularly when producing a large number of discs, the discs are preferably piled up to be dried. However, if the discs are not sufficiently dried when the discs are piled one on another, surfaces of the adjacent discs may stick together. Hence the efficiency of the operations thereafter is reduced.
Referring to FIG. 5, in order to prevent the sticking of the surfaces, there is provided annular projections called stack rings 42 formed on both surfaces of a CD 4. The standard radial position of the stack ring 42 is set at a position between 16.5 mm and 22 mm from a center O of the CD in radius, and the standard height of the stack ring is 0.4 mm from the disc surface at maximum. Actual design of the stack ring such as the width and the position in the radial direction of the CD is arbitrarily determined by the manufacturer provided they are within the standard. When the stack rings are provided, each CD touches the adjacent CD not by the entire surface thereof, but only by the ring. Thus the sticking of the CDs can be prevented.
There is known a multifunctional disc player capable of additionally playing a disc besides the CD such as a video disc. The video disc has a diameter different from that of the CD. In addition, the centering hole of the video disc which engages a centering hub of the disc player for positioning the disc is different from that of the CD in diameter.
FIG. 4 shows a centering mechanism of such a multifunctional disc player for accurately positioning the rotational center O of the disc at an axis X of a spindle motor of the player. The structure of the mechanism is symmetrical with respect to the axis X so that only one side is shown in the figure.
Referring to FIG. 4, the centering mechanism comprises a centering hub 3 and a turntable 2 both of which are securely mounted on a shaft 1 of a spindle motor. The centering hub 3 has a center portion 3a which engages with a centering hole of a small diameter disc, namely the CD 4, and an outer portion 3b which engages with a centering hole of a large diameter disc, namely the video disc 200 (FIG. 3). The upper outer periphery of the center portion 3a is cut away to form a conical periphery 31 and the upper outer periphery of the outer portion 3b is also cut away to form a conical periphery 32. A spring 5 is provided between the turntable 2 and the centering hub 3, thereby urging the centering hub 3 upward.
An optical pickup (not shown) for reading out information from the disc is generally provided on the same side of the disc as the turntable 2.
As shown in FIG. 4, when the CD 4 is mounted in the disc player, the peripheral wall of a centering hole 41 thereof engages with the center portion 3a of the centering hub 3. Due to the urging of the spring 5, no gap is formed between the CD 4 and the conical periphery 31 of the hub so that the CD 4 is accurately positioned. At the same time, the CD 4 is urged downward by a damper (not shown) against the turntable 2.
Referring to FIG. 3, when the video disc 200 is mounted, the conical periphery 32 of the outer portion 3b engages with the inside wall of a centering hole 201 of the video disc 200 without forming a gap between the disc 200 and the conical periphery 32 by the spring 5. The video disc 200 is also urged by the damper against the turntable 2.
The standard clamping area of a regular disc player for exclusively playing the CDs is set at a position within a range of 13 mm to 16.5 mm from the center in radius, that is, 26 mm to 33 mm in diameter. However, since the centering hole of the video disc has a radius of 17.5 mm, the clamping area falls within the centering hole of the video disc, rendering it impossible for the turntable to hold the video disc. The turntable for the multifunctional disc player must be determined so as to be positioned on the outer side than in the regular disc player.
On the other hand, the information recording area of the CD starts at a position 23 mm from the center in radius, that is 46 mm in diameter. If the size of the turntable is increased, it will block the inner portion of the recording area, thereby preventing the reproduction of the information. Hence the width of the holding surface of the turntable is inevitably limited to fall within a range between positions 17.5 mm and 23 mm from the center in radius. As a result, the turntable must hold the CD at the stack ring, which is formed in a range between positions 16.5 mm and 22 mm from the center, as shown in FIG. 5.
As a result, the clamping height, which is the distance from the holding surface of the turntable 2 to the recording surface of the disc, varies from disc to disc due to unevenness of the height or the width of the stack ring 42. Moreover, since the disc is raised from the turntable a distance corresponding to the height of the stack ring, the disc cannot be sufficiently clamped, thereby increasing the amplitude of the fluctuation of the disc. As a consequence, disturbances which are not anticipated by the set standard occur. Thus a servo system must be designed so as to be able to restrain these disturbances, thereby increasing the dimension of the circuit and hence the manufacturing cost.